The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In a general well cementing operation, a cement slurry is prepared at the surface and pumped into the well to fill the annulus between the casing and the borehole wall to provide zonal isolation and mechanical support. Interest in the recovery of heavy oil is increasing because of the huge reserves of heavy oil existing whereas reserves of light oil are decreasing. Thermal recovery processes are one of the main means of recovering heavy oils. Thermal methods work by increasing the temperature of the oil which will decrease the viscosity of the oil. One of the main thermal methods used is steam injection. However one of the issues for zonal isolation is the permeability of cement after steam injection operations. When initially set, the cement can provide a good seal, however changes in pressure and temperature during repeated steam injection procedures can induce stresses and affect the integrity of the cement.
Systems used for such application are typically conventional low density cement which become highly permeable after a few steam injection cycles. It is common that temperatures around 300° C. are reached within the cement sheath and generally the set cement will lose strength and gain permeability after regular steam injection. This can cause zonal isolation to be lost and induce cored casing and/or steam leakage. As such the lifetime of thermal recovery wells is shortened by all these stresses the cement is placed under.
Lost circulation problems often occur due to weak and unconsolidated formations of such wells therefore low density cement systems are required however this is detrimental in terms of set cement properties, as water permeability of set cement is inversely proportional to the density of conventional cement. Heavy oil wells exposed to thermal recovery processes belong mainly to the low tier markets where low slurry pricing is a key point to be competitive.
One cement system developed, FlexSTONE (Schlumberger), maintains high compressive and tensile strengths compared to conventional cements, and is used in steam flood operations. However the permeability of the cement is still a problem at 300° C.
Therefore the object of the invention is to provide a cost effective additive to increase the long-term permeability performances of set cement.